Recently, progress of technique on a laser has been remarkable, and particularly, a solid state laser or semiconductor laser having an emission range of from near infrared ray to infrared ray is very useful as a light exposure source in a system wherein a printing plate precursor is directly imaged by digital data, since such laser of high powered and compact type has been easily available.
As an image recording material suitable for laser-writing, for example, Japanese Patent Un-examined Publication (hereunder referred to as “J. P. KOKAI”) No. Hei 7-285275 has suggested a positive-working image recording material comprising (a) a binder such as a cresol resin, (b) a compound which absorbs light so as to generate the heat, and (c) a compound such as quinonediazide which is heat-decomposable, whose pre-decomposition state being capable of substantially decreasing a solubility of said binder. The mechanism of this material is that on IR radiation, the light-exposed area becomes alkaline-soluble, since the compound (b) generates the heat in the light-exposed area and then the compound (c) is decomposed (i.e., heat-mode type). However, the generated heat is absorbed in some degree by a substrate aluminum plate and so the thermal efficiency of this mechanism is poor, and therefore the solubility of the light-exposed area to an alkaline liquid developer is not satisfactory in a development process. Consequently, alkaline strength of liquid developer must be raised to ensure the solubility of the light-exposed area.
On the other hand, in the lithographic printing plate precursor of heat-mode type, the resistance of non-heated areas (i.e., image areas) to dissolution in an alkaline liquid developer is poor under the above highly alkaline concentration, and when the image recording material has slightly scratches on the surface thereof, said scratched areas may be easily dissolved. Accordingly, there is a problem such that the image areas, particularly fine line areas and the like are easily impaired. This tendency is particularly serious in a positive-working lithographic printing plate precursor employing a polymeric compound which is highly soluble to an alkaline aqueous solution.
Consequently, the raise in alkaline strength of liquid developer should be limited, though said raise in alkaline strength is intended not to leave undissolved residue on non-image areas, and it is also difficult to form a highly sharp and clear image without impairment to the formed image areas. There is therefore a need for improved sharpness and improved reproducibility of images, particularly fine images comprising a dot pattern, fine lines and the like. For the purpose of meeting the need, addition of various surfactants to a liquid developer has been researched, and this attempt can attain an effect to some extent. However, it becomes a subject of discussion that the performance of the surfactant is decreased through dissolution of components of a light-sensitive layer into the liquid developer.
Incidentally, in association with recent improvement on image recording materials of IR-laser exposure type, it tends to use an IR-absorbing agent such as a cyanine dye in the image recording material, which absorber is more insoluble in an alkaline liquid developer than a conventionally used IR-absorbing dye. When such image recording material is processed in an alkaline liquid developer, insoluble matter originated from the IR-absorbing agent possibly appears in the developer, and such insoluble matter may interact with a component of binder polymers in the image recording material or inorganic substances in water to generate further insoluble matter. The above insoluble matter may adhere to a plate during development procedure in making a printing plate, and then image areas of the plate may be impaired. In addition, there is a large labor or cost for maintenance of processing tanks, since it is necessary to regularly remove the precipitated insoluble matter in the processing tanks.
Thus it has been required that the disadvantages by the insoluble matter originated from the IR-absorbing agent and the like are eliminated, a highly sharp and clear image is formed without impairment to the formed image areas, and sharpness and reproducibility of images, particularly fine images comprising a dot pattern, fine lines and the like are improved.